Seismic wave filtering system



July 31 1956 R. A. PETERSON SEISMIC WAVE FILTERING SYSTEM 4 Sheets-Sheet l Filed Aug. l2, 1953 TTQQNEX QA yMo/VD A. Pers/so. I IN VEN TOR.

July 3l, 1956 R. A. PETERSON SEISMIC WAVE FILTERING SYSTEM 4 Sheets-Sheet 2 Filed Aug. l2, 1953 E w b Q u Q 1N V EN TOR.

rroeA/ex July 31, 1956 R. A. PETERSON SEISMIC WAVE FILTERING SYSTEM 4 Sheets-Sheet 5 Filed Aug. 12, 1953 Mas.

f lgli INVENTOIL 19A wvo/VD A. Per-52 so/v,

July 31, l956 R. A. PETERSON 2,757,357

SEISMIC WAVE FILTERING SYSTEM Filed Aug. l2, 1953 4 Sheets-Sheet. 4

.34m/:OMD .4, Perk-eso@ INVENTOR.

y 2,757,357 J sEIsMIc WAVE FILTERING SYSTEM "1 RaymondnA, Peterson, Altadena, Calif., assignor, byl

. mesne assignments, to United Geophysical Corporation, "Pasadena, Calif., a corporation of California 1 Application August 12, 1953,Serial No. 373,820 '15 Claims. y(Cl. 340-15) This inventionl relates to seismic prospecting and more" particularly to improvements in systems which involve ception points.

'In reflection seismic surveying as generally practiced; seismic waves are generated by detonating `an explosivey of the earth. The" downwardly throughf successive subterranean lstrata and small yamounts of the" charge at a point adjacent the surface seismic waves so generated travel energyin the seismic waves'are'freflected upwardly-by the successive strata. The reflected' Waves that return to the surface of the earth from the various strataare detected there by means of geophones, or seismometers,

that are located at various seismic wave receiving-sta`4 tions located at or adjacent the surface of the earth. At`v the same time random seismic disturbancesfintheu na-' ture of noise" also affect the geophones. Such noise is due in part lto the eiect surface, in part to the diffraction and scattering of the seismic waves from irregularities beneath the surface of the'earth, 'in p'art to surface waves generated by the eX- plosive charge, and in part to spontaneous disturbances arising within the earth or otherwise. The reflected seismic waves and noise cause the geophones to'vibrate thereby producing at their outputs corresponding elec- After suitable amplifcation, filtering, orl other treatment, the electric waves generated by the re-- trical waves.

spective geophones are generally recorded as separate traces by means of a multiple elementvoscillograph.

As is well known, the time required for seismic Waves' to travel from a shot-point to the various reception points is `employed together with other data including measurements of lthe spacing of the various reception points from each other and the velocity oftravel of the to determine the depth and the strike and dip of reflect'- ing strata. By making such determinations at various positionsl throughout an area under investigation information is obtained as to the structure and even the character of subterranean strata thus providng termining whether to locate an exploratory well or other Well yin the area that is undergoing investigation or development, and if so where best to locate such a well.

The effectiveness of a reflection seismic surveying technique depends in part upon the ease with which reliected waves may be identified in the dilferent traces of the recorded seismogram and also in part upon the accuracy with which the times of arrival of rellected waves at the various corresponding geophones maybe measured. Both the f recognition of reflected seismic Waves and the measurements of their times of arrival at various corresponding geophones are important in the performance of a successful seismic survey. The reception and recording of noise along with the retlected seismic' waves rei duc'es the ease of recognizing the reflected seismic waves in the various traces and also reduces theaccuracy withl which their times of arrival may be measured on the traces. For this reason in practice, the electric waves, generated by the geophones are frequently passed throughl of wind on vegetation at'the seismic waves;

the-"reception of reected seismic wavesat a series'of rely those which do not characterize of'ce'rtain legs in shapes of other legs are dissimilar and zin scription taken in connection Patented July si, s

frequency selective lters in order to attenuate'some of: i

the unwantedjiioise lcomponents compared with the desired reflectedl waves; Y y

It is observed that when `seismic waves are generated and detected inthe manner described above, the simplest or half waves. When such a wave is passed throughc a filter, its formis modified, frequentlywith an increase in the number of legs orv half Waves. The total "duration of a liltered wave is generally longer 'than the dura-` phones. to the fact that'when an electricalwave seismicwave is ypassed through different filters, the shapes the outputs are veryvsimilar, while the any event the over-all appearance of the outputs of the different iilters are similar in Y i Furthermore, similar legs in the outputs of are usually displaced from each other in time.` y

According tothis invention, the'outputs vof the filters associated with each geophone are combined and a corrective "time displacementv is introduced prior to combining sok that selected similar legs or parts ofthe outputs coincide in timeand hence their effects become enhancedv in the combining process. parts into coincidence prior legsA or parts are rendered tend to nullify each other elfect the selected legs or parts are brought intolphase while the other legs orparts are ldephased prior to ,combining. As a result of this operation the selected legs or parts of the seismic wave are enhanced greatly Iin the lcombined output compared to the other vparts' of the seismic Wave, thus, in eiect, tending to shorten the dura- 4to combining, the dissimilar -tion of Vthe output and to facilitate recognition ofl the- .v

selected leg or part. Also according. to this invention the lilters employed are so selected that they tend'to emphasize certain frequency components of reilectejdseisinicV waves compared to other frequency 'components especial` reilected 'seismic waves but characterize only noise. For this reason and also because the noise signals that ing through the lters, the amplitude of the noise coinponents in the combined output is reduced compared to the amplitudes ofthe v sult of this invention, the signal-to-noiseratio is increased in the recorded traces thereby l tion of reected waves and increasing the 'accuracyfof together with various objectsV and featuresvthereof will become apparent from a reading drawings wherein:

Fig. 1 is a schematic diagram of a seismic wave re-` ceiving and recording system employed in this invention; n

a seismic wave re- Fig. 2 is a schematic diagram of producing system employed in this invention;

their times of arrival at lvarious geo- The effectiveness of this invention is due in part part but widely different in other parts;

dilerent filters IBy bringing similarV legs or.

noncoincident so that they'. in the mixing process. .In`

are characterizedfby such other frequency components tendto bey dephased in passj selected legs or partsg As a re-" facilitating 'the recogniof vthe followingde-qk with the accompanying Fgl 3t is` a sheni'itic diagijanluofa system for adjusti' ps'ti''s" of the' thsducing heads;

Fig. 4 is a series of graphsfjemployed in explaining the of the.ini/entiere.,4 ,1 1... .s s, islswg `entof `eisrrbizograrn`of` the type protheinventioni. t 1 matiediesreoifenothef.. ding system employing this' oo- Salln; '-91.1 r1. L' Sseheens.hvnrrennlsnti the eft- The venous eonhon Ri theerounsi. into Outputs.

A.. e. L., i i... eouesvondue eutometievolufn .oonintendere. i

til?

arranged;

n tthe. gehphhnestt L insthe@ tthe institut oi.

one brest einen he .eheise' of .explosive fee .51 Ys e. s sinne! eenefetedhenyseill tor Vene-us eet111es9i..s.11eh.ie svstenli hevelhsenidnelosed so the s'arrie heads kbeing employed `recording. The outputs sponding iters signets. are. trensforrne.d to4 weve n the menetic tape;

l each'A set .have

i i. adjustmentsgtp', headshkiand hi for reproduclng as for of 1 the respective demodulators d"2 are transmitted through correfi, 3"'1, fz,rfz some of which f have one frequency characteristiqand some of which f" have diierent frequency characteristics as explained more fully hereineften A..n.lurelitv.. gengedntyvolrposition switches s1,` 5"12'52,l s2 Jarranged at the outputs of the' resse ,ifeisj'lgf magasiner-x* seele the tiene te es essere@ galvanomt'e'r elients ."of ifl iple elementr .oseillogrerlm` or .to `freed `the; outputs...toA .oombinng means k1,y k2 that corresnondurespectively to the geophones R'li Rewrs'ini switches S'r, S1, S'z, S"z are arranged between the outputs of the respeev tive lters f'r, 3"'1, z, 1"2 y. and the corresponding tv'vehosition-svvitehes. s1,ts'.1,..s2,.sf.2

oder...t.o .repro s J.. s. m4 t ,duce .oseillographie giorni. `',th.e.. reeoid that have .been recorded.` .on various.. .freeler 4magnetic tape T is driven at constant speednbyI th groter Moest. the .various heads. 1.111,1A hf'i, hielt. on .t.i.1er.ehy

- eeusins tiorresnonding....frequenev .modulated .vl/aves to. epreermetrtheir outoULs..

thos. .thetleppeenet the outputs of.`the; eo'respo modulatenoseilletorsn.O24.. O6-, During Passage. through he deinodletors. dfi., .d"1,.;..l.'n;.dfi G the. forms. inv/h .11. th..- emplitnde. of. thevoltege. verles. .withti.rne-in `.the sente wey;..the.t..the.;velooitv;.otthe eorrespondng geophonos, varied `w.itl.1...tixn.e.in response .to .the .arivelof soient?. waves; teltins into eoeountfof. course, `the volume `control. aetionofihe AYQeniplie'rs-A1A2..- Ao. .Thus .ai the. outputof eaehp hirof .dernoduletorsd' .and d".e.0rrem spending to. pertieule, geophone- R. there. produced apeirtidentieal sinnnlsrepresenting .the seismic nieves. reeeivediby .the eotresponding.geophone These signels may he. displaced.. relative. to .eeeh` other. in.. time; hvdisr. pleeingithe ...corresponding reprodueins- .heads ,relative to each other .along u .mesnetio tope,

.thelentb of the.

.Asirnple l errengerne.. t for displacing` 4the .,lrepnod'uoins. heads, hothmindvidnally and in .setselongtthe length of` .s. illustretedinFig. 3.-; Here, .it vviilbfe. noted. ,thatgthe trensdueing .heads hi, h.'2 ofoneset are. .adinstablrrositioned ,relativeto e bei. blendihet. the otteqtrensdueinsheedshi; hf'e.- ofwhe. other. sei. areadiusteblr positionedrelet'ive toe henbf. oodthetthe two. .bars bgah'.. are movable. relative toeeoh Jtrillen.. .with

this. errarurernent, thel .transitions olf. the.y individus ends. oieeehs'einiey he adiustedinenv.wes/.desired letive to..,ot.her...heedst.in the. same. set, to'` rneley--weatlheringb c orrectionsor for. other pnrpo'ses,,Qnee .theheedsiiin been .snitablvrositioned rel.etive`...tothe bars; ein` whinithey aremounted. the...t.wo sets. ome-eds. menthe. adjusted in unison,` onereletive.fo.the.othe inf. order..rohdispace-signals applied to the `notrespon ns serskof .filters .relative4 to. each ether- .vopolvingequei corresponding toreeeh..

the ms aA aneous b n m corresponding frequency" the f n v'vaves. .which .are to.

be detected.; .The oessuhendso two sets frlters. generally `over r, sometimes e ingmnemithinrhe other.; infie-1.4 the curves thatfindcate. .how ..themlters` tra sismiq wave. transmitted .therethrohgn whereas; Eier thlbandpass characteristics of the iters referred to are indcated l M l .t .,-;..J .Wheir atsirnple" seismic' wave having three iegs erhalt` Waves as lrepresentedby graph w of Fig. 4 is passed through a filter f' having a. characteristic represented by the 'graph g', the resultant output has the appearance of a wave represented by graph w'. When the same seismic wave is transmitted through a filter f" having a characteristic represented by the graph g, the resultant output has a shape represented by the graph w". It will be noted that the waves appearing at the outputs of both filters are of longer duration than the wave applied to their inputs and that the output wave forms are more complex than the input wave forms, the output waves in fact having more legs or half waves. However, it will be noted that certain selected legs or half waves have a high amplitude and are readily correlatable. For example, in the particular examples shown the leg l' of wave formw and the leg l of wave form w both have high amplitudes and are of a similar shape and duration. It will be noted, however, that the maximum values of these selected legs do not occur at the same time, but each is delayed by a different time interval relative to the time of application of input wave form w. rIn fact it will be noted'that the maximum leg of the output of the filter f lags the maximum leg of the output of the filter f' by a differential time At.

According to the present invention, the reproducing heads hi and h1 are displaced relative to each other by such a distance as to introduce a time lead between the signals applied to the demodulators d'1, d1 in such a direction as to cause the two corresponding selected legs of the waves w' and w" to occur simultaneously at the outputs of the filter. Then to accentuate part of the wave, these outputs are applied to the combining means k1. The combining means may be in the form of a circuit which algebraically adds the outputs of the filters f and f or it may be in the form of a multiplying circuit or other convenient combining means. The result produced by additively combining the outputs of the filters f' and f is illustrated in graph wk of Fig. 4. Here it will be noted that though the combined wave wk is of slightly longer duration than the input wave w, in effect, because of the bunching of the energy in a short time interval, the combined wave appears to be of shorter duration than the waves appearing at the outputs of the filters f and f. Furthermore, certain parts are more highly accentuated relative to other parts than are the corresponding parts of the waves that appear at the outputs of the filters f and f".

In one way of practicing the invention the heads hi, h"1, h'2, h are aligned transversely of the record T in the same positions that they were in during the recording. Then with the ganged switches si, s"1, s'2, sz set in their first positions the motor M is operated, driving the magnetic tape T past the heads. As the various record tracks move past the heads, waves corresponding to the seismic waves that were originally detected by the geophones are reproduced at the inputs of the corresponding filters. Thus waves corresponding to the seismic waves received by geophone R1 are reproduced at the inputs of filters fi and f1 and waves corresponding to the seismic waves received by geophone R2 are reproduced at the inputs of filters fz and f2 After passing through the corresponding filters fi, f1, fz and f"2 the seismic waves are recorded as oscillo- 'graphic traces by means of the corresponding galvanometers g'i, g"1, g'2, g"2 thus producing a twelve trace record, two traces corresponding to each of the geophones. The resultant seismogram is then examined to identify reflected waves in some portion thereof which it is desired to study in more detail. Once such a reflection has been identified, the time displacement At between selected parts thereof on traces corresponding to different filters is measured. This time displacement is a characteristic not only of the pair of filters employed, but also of the shape of the particular reflected wave which has beeniidentified and the relationship of the legs that are to be synchronized. Then the heads are displacedrelative' to eachother'byl amounts that are required to offset the observed time displacement. V-

In the selection of the parts or. legs-.which are to'be4 emphasized, it issometimes found that the trace corresponding to the output of one of the filters will more nearly correlate in the selected .region of a transient with the traces from the other filters if its polarity or phasey is reversed. Advantage is taken of this fact by employing the reversing switches Su, S"12, S'zi and S"22 Thus reversing switch S11 provides means for reversing the polarityor phase of the output of filter f'i, reversing switch S12 provides means for reversing jthe polarity or phase of the. otuput of filter f1, etc. When it is found that reversal of the polarity of the output of one of the filters will increase the correlation between the selected parts orlegs of its output and the corresponding parts or legs of the outputs, from the other filters, they reversing switch associated with the firstnamed filter is operated. Then the relative spacing of the' reproducing heads isvadjusted to bring the selected parts or legs into coincidence. pl, p1, pz, p2 are employed after the switching between the outputs of the corresponding filters fi, f"i, f2,f2 and lthe input of the combining means k1, k2 in order that the waves may be combined in any desired proportions. By suitable choice of such proportions, the parts of the waves which are not to be emphasized may be minimized in amplitude.

With the heads set to compensate for the time displacement introduced by the filters, the ganged` switches si, s1, sz and s2 are set in their second position thereby applying the outputs of the filters to the combining means. Under these conditions, when the magnetic tape is again driven past the recording heads, the outputs of the combining means are recorded as corresponding traces by means of the corresponding galvanometers g1, g2 A section of a final record produced by the system showing six corresponding traces is illustrated in Fig. 6. Here it will be noted that reflected waves W1, W2, W3 We are recognizable not only because of their high amplitude compared to the noise present in adjacent parts of the record, but also partly because of the regularity of the time displacements between waves recorded on successive pairs of adjacent traces. It will be noted that this record also exhibits timing lines t. These lines are produced on the seismogram by translating the signals recorded on thev timer track of the magnetic tape into a timing signal by means of a reproducing-head and demodulator, andl applying this signal to a time-line generator G forming part of the oscillograph.

It is not necessary, of course, to determine the displacement of the heads that is required for each record produced in an area. In the usual practice, it is found that the shape of reflected waves received in a particular part of a record is uniform over a wide area. In this case tables may be made which set forth the values of the displacement as a function of the time of the recorded section and as a function of the sets of filters'that are employed. Once having determined such a set of values for a particular area, it is then only necessary to select filters for use in producing combined records of waves that arrive in a particular time interval after the detonation of the charge of explosive and to set the heads at a corresponding position. Then even though no refiection at all may be identified in the portions of particular records which are undergoing investigation when the records are reproduced without mixing, the signal-to-noise ratio of refiected waves that exist in such portions `of the record will be increased by translating the seismic waves into corresponding electrical waves, passing these waves through filters having different characteristics and combining their outputs and making records thereofas explained above.

By selectively amplifying electrical wavescorrespond- In addition, potentiometers.

the; utpu as av 'separate sin `T. Such a system issubs't`antially identical withthat illustrated in Fig. l, except that'only one recording head `h1 h is `employed to record the output -produced `by yeach "of the corresponding fseismon'ieters R1 R6. Thus, s illustratedin Fig. 7, the output 'of e'ach seismoreter R1 Rs is passedlthrough a 'cdrrsponding input channel IC1 lIC@ Vto a correspondingrecordinghead h1 11s. Each 4of the 'input channels 1C includes an ampliier A and "a frequency oscillator "O ias villustrated in Fig: l. channel ICO comprising an oscillator "O, 'an amplifier An, and a modulated oscillatorOo `as illustrated in Fig. l is employed to rcord the Atime -brelc and `the 4timing line r timing wave. Thus, with 'this arrangement, seven R isrecorded magnetic tape trcks'are irecorded on the magnetic ltape T, `one Arepresnting the timing Wave and six :representing the outputs of the 'six seisrri'orneters R1 Re. p

`In1:ig.x8,`there is illustrated anrrangement fo'rrepro'- ducing the signals 'recorded on the *magnetic tape in accordance with the system of Fig. 7. In this reproducing system, two sets 'of Vtransdueirig heads are `employed. One set'oftransdu`cin'g heads 11"'1, "h'z are -mounted on abar b'; Likewise, the second `set'of transducing heads h"1, h"2 u of a second 'set are mounted on asecond ba'r b. Thisarrangement-is similar to thatiillustratedin Fig. 2; vexcept 'that `in this case one head `df each 1set `is lo'cated opposite the same tack on the magnetic tape T. Thus, lthe two headshi, h1 are aligned with'and located oppsitetlie'trackon which thefutput frm'tlie seism'meter R1 is recorded. h"2,-h" are aligned with and tilhichith nntpuffi;dfnthe? Tlie "outputs f`ot' I'aleli jpair If Likewise, an input In a -slniilfar way, the Itivo heads 'appaian- Yto" the includes` dei* flue:k

with those predire l needle-tenth ci he output: E

l'tin, r lI Q'this iasethejc i e are il assed thro 'wkstth'e'tefs fio g'apipliaf" tputs' to 'the' feombmnig feirc'uit's.: thlereforetobe' understood that the vinventionis not to the :specific `eml/odiment thereof which has been `described abovebut Eis capable of lbeing embodied `in many dierent forms Within the scope of `the appended claims.

What is elaimedfis:

l. In apparatus for seismic prospecting: a geophne='for receiving lseislmic 4waves arriving at a point Ain the earth and `forment/erh'.'ug said Aseismic `i/vavc-:s-into corresponding electrical Waves; means =or translating the outputofsaid geoplibne into -a plurality of asimilar 'trainsof `electrical Waves that'aredisplaced in time `from each other; a plurality tof lltering` reans having idie'ren-t frequencyresponse characteristics; :means for applying lthe time displaced electrical waives to `the inputs of isa-id 1liltei'ing meansgmeanstoi-'combining the outputs of saidtiltering means; and means for=recording `the combined outputs as a "function of time.

fln apparatus yfor seismic prospecting; algeophone -for receiving seismic waves arriving at `-a point in thefearth and forconverti-ng-said seismic waves into corresponding electrical waves; rr'i'eans, Vfor translating Y the output of said gephone into afplurality-of similar trains of `'electrical waves that are-displaced'intime-from each other', means for varying the time displacement between said `wave trainsaplurality -of ltering means-having dierent frequenc-y-response characteristics; means for applying the time-'displaced electrical WavesV to the inputs of said iilt'eririg means; means for combining the 4outputs of-said filtering meansyand 'means for recording the combined 'outptits "as a Afunction 'of time.

filtering means While displaced geophone in to a plurality of similar trains of electrical waves; a plurality of filtreing means having different frequency-response characteristics; means for applying the respective trains of electrical waves lto the inputs of corresponding filtering means; means for separately recording the outputs of said filtering means as a function of time; means for displacing said electrical wave trains relative to each other in time prior to application thereof to said inputs; means for combining the outputs o f said electrical Wave trains are applied to their inputs; and means for recording the combinedoutputs as a function of time.

I '4. In a method of seismic prospecting in which seismic waves are generated at a shotpoint and are then received at a series of reception points that are spaced apart fromA each other and from the. shotpoint, the received waves including noise and also reflected waves, the steps of: translating the seismic waves received at each reception point into corresponding electrical waves; ltering the electrical waves corresponding to each reception point in a plurality of different ways whereby electrical waves are produced that are similar in part and dissimilar in other parts; and combining the filtered electrical waves corresponding to each reception point to additively combine similar parts of the waves wherbey a combined wave having a part of increased recognizability is produced.

5. In a method of seismic prospecting in which seismic waves are generated at a shotpoint and are then received at a series of reception points that are spaced apart from each other and from the shotpoint, the steps of: translating seismic waves received at each reception point into corresponding electrical waves; selectively amplifying the electrical waves corresponding to each reception point in a plurality of different bands whereby selected frequency components of the waves are emphasized relative to other frequency components; and combining the amplified electrical waves corresponding to each reception point to additively combine the selected frequency components whereby the recognizability of parts of said waves is increased.

6. In a method of seismic prospecting in which seismic waves are generated at a shotpoint and are then received at a series of reception points that are spaced apart from each other and from the shotpoint, the ing seismic waves received at each reception point into corresponding electrical Waves; filtering the electrical waves corresponding to each reception point in a plurality of different ways; emphasizing selected components of the waves in all the filtering operations; and combining the filtered electrical waves corresponding to each reception point to additively combine the selected components and to subtractively combine other components whereby the recognizability of said selected frequency components is increased.-

7. In apparatus for seismic prospecting: a plurality of geophones for receiving seismic waves arriving simultaneously at various spaced points in the earth and for converting said seismic waves into corresponding electrical waves at their respective outputs; means for recording the outputs of said geophones as corresponding reproducible records along the length means including a plurality of reproducing heads associated with each geophone for converting the waves recorded on the record strip into corresponding electrical waves, whereby electrical waves corresponding to seismic waves received by each geophone are produced; filtering means connected to the output of each reproducing head, the filtering means connected to heads associated with each geophone having different frequency characteristics; means for relatively displacing the heads corresponding to each geophone along the length of the record strip, whereby the latter electrical waves corresponding to each geophone are displaced in time relative to each other by such an amount as to bring selected parts of the filtered outputs associated with each geophone into phase with of a record strip;.

steps of: translatt each other; means for combining the outputs of filters' associated with the respective'geopliones; and means for recording the respective combined outputs as separate functions of time having a common base whereby the relative times of geophones may be ascertained;

8. In apparatus for seismic prospecting: apluralityv of geophones for receiving seismic waves arriving simultaneously at various spaced points in the earth and for converting said seismic waves into corresponding electrical waves at their respective outputs; means for recording the outputs of saidngeophones as corresponding reproducible records along the length of a record strip; means including a plurality of reproducing heads associated with each geophone for converting the waves recorded on the record strip into corresponding electrical Waves whereby electrical waves corresponding to seismic waves received by each geophone are produced; band-v pass filtering means connected to the outputof eachv reproducing head; different filtering means connected to heads associated vwith each geophone having overlapping pass bands but different cut-off vfrequencies;'means for relatively displacing the heads corresponding to each geophone along the length of the record strip whereby the latter electrical waves corresponding to each geophone are displaced in time relative to each other by such an amount as to bring selected parts of the filtered outputs associated with each geophone into phase with each other; means for combining the outputs `of t1lters-asso-` ciated with the respective geophones; and means forfrecording the respective combined outputs as separate functions of time having a common base whereby the relative times of arrival of such seismic waves at the geophones may be ascertained.

9. In apparatus for seismic prospecting: a pluralityof geophones for receiving seismic Waves arriving simultaneously at various spaced points in the earth and for con-l verting said seismic waves into corresponding electrical waves at their respective outputs; means for recording the outputs of said geophones as corresponding reproducible records along the length of a record strip; a plurality of sets of reproducing heads associated with said geophones, one head of each set being a-dapted to convert the recorded Waves from a corresponding geophone into corresponding electrical waves, whereby electrical waves corresponding to seismic waves received by each geophone are produced; a plurality of sets of filtering means connected to the respective sets of heads, the filtering means of each set having similar frequency-response characteristics and the filtering means of different sets having different frequencyresponse characteristics; means for Idisplacing the sets of heads relative to each other in unison along the length of the record strip whereby the waves from each geophone are reproduced as a plurality of similar trains of electrical waves that are displaced in time relative to each other by such an amount as to bring selected parts of the filtered outputs associated with each geophone into phase with each other; means for combining the outputs of filtering means connected to heads associated with each of said geophones; and means for recording the respective combined outputs as separate functions of time having a common base whereby the relative times of arrival of such seismic waves at the geophones may be ascertained.

l0. In apparatus for seismic prospecting: a plurality of geophones for receiving seismic waves arriving simultaneously at various spaced points in the earth and for converting said seismic waves into corresponding electrical waves at their respective outputs; means for recording the outputs of said geophones as corresponding reproducible records along the length of a record strip; means including a plurality of sets of reproducing heads associated with said geophines, one head of each set being adapted to convert the recorded waves from a corresponding geophone into corresponding electrical waves, whereby electrical waves corresponding to seismic waves received by each arrival of such seismic waves at the affres/eer geopifone aire lproduced; .aplurality' of Vsets of filtering means `connected totherespective sets of` heads, l:the .liltering-means of feaoh set yha'vingrsimi'lar frequency-response characteristics `and the filtering means of l different sets having diterent frequency-response Characteristics; means for displacing the sets of heads relative to'each yother rinunion yalong the Vlength `of the record fstrip -whereby the faves from each Vgeophone` are reproduced as a plurality offsinilar'trainsrof electrical waves that `are displaced in t-ime=relative to` each other fby nsuch fan amount as =to bring selected parts oftheltered outputs 'associated with each geophone Ainto tph-ase with i each other; means 'for combin- .irtgthe :outputs of `iiltering means connected -to rheads associatediwitheach Aof said geophones; and ineans for recordingthe respective combined outputs assepa'rate *functiorisfof time .having iacommonbase whereby :the relative tiineslofsarriv'al of such seismic Waves at `the geophones may berscertained.

'-ll; vIn apparatus for seismic :prospecting: a plurality of geophones `for 'receiving Vseismic waves arriving `simultaneously" 'at various #spaced points in the vearth and foi-*conueiiting said seismic waves into corresponding electrical waves-at'their respective outputs; means for recording the otitputof 'each ugeophone as a plurality of corresponding reproducible records along the length of 'a `record strip; a plurality yof :reproducing 4heads associated `with each geo phone for `converting lthe waves recorded on the record strip into corresponding electricalwaves; means for moving :such :r'e'co'rd strip 'past said lreproducing'heads whereby electrical waves corresponding to seismic waves received by lech .geophone are produced;ltening meansc'onnected to Tthe Voutputirt" each reproducing head; diierent filtering means connected toheads associated with each geophone having different frequency characteristics;`rneans for relatively displacing ftheheads corresponding to eachgeophone Valong 'fthe length of `the record strip, whereby Athe latter electrical `waves corresponding to each geophone are dis placed in time relative to each other 4by such yan amount as to bring selected parts of the liltered voutputs lrassociated nwith each geophone into phase Awith eachother; nean's for `combining `the outputs-of lters associated with the respective .geophones; `and means `for `recording ythe respective-combined outputs ras separate unctionsoftirne having a -comrnon base whereby the relative times of arrival of :such seismic Waveslatthe geophones'may be ascertained.

l2. In `apparatus for Vseismic prcspecting: a pluralityy of geophones for-receiving seismic waves arriving `simultaneously at various spaced .points `in the learth and for converting said seismic'waves into corresponding-electrical waves at their respective outputs; means `including Latpluralitiy of transducing heads '-for 'recording theoutput `of each fgeophone as a-plurality of corresponding 'reproducible records along the length of a record strip; filtering means `connected Ato said transduc'ing'heads 'for converting the waves recorded-on the'record strip into corresponding electrical zwaves; theflltering `means connected `todierent heads associated lwith eachgeophone havingcorresponding different =frequer1cy characteristics; means for `relatively displacing the heads corresponding to each geophone along the length `.of the record-strip whereby the wavesffromeach geophone are reproduced as a plurality of .similar electrical 12 waves that are displaced in time relative toeach'othcr -by` such an amount" as to bring selecte'rlparts of the filtered outputs associated with each ygeophone into phase with each other.; :means `for lcombining `the outputs fof-filters associated with the respective geophones'; `and 4means `for recording the respective combined outputs -as1 separate f-unctionsof `time having a common base whereby the rela' tive `times of arrival of such seismic waves at the geo-` phones `may-be ascertained. l 13h. VIn `apparatus for seismic prospecting:` `a geophone for receiving seismic waves arrivingat a -point lin `the earth and for converting said seismic waves into-.conce spending electrical waves;` a plurality of `iiltering means having ,diterent frequency-response characteristics; means for applying the electrical waves in parallel to the inputsof said filtering means, whereby filtered ysignalswha'ving" different shapes appear at the outputs of said Vtiltering means; adjustable time delay means :for relatively` delaying` the filtered signals by such `amounts as to` bring selected parts of 'the `filtered outputs into phase with ea'ohl-othet; means for combining the outputs of sa-id iiltering means, and means for `recording the combined outputsas a iunc-` tion of time.

14. 4In apparatus for yseismic prospecting: aogeophone tor receiving seismic `waves arriving fat a point inlthefearh and Afor `converting said seismic waves finto corresponding electrical waves; aip'lurality `of filtering means `having different frequency-response characteristics; means 'for -apw plying theelectrical waves yin parallel 5to the inputs of said filtering means, whereby filtered signals [having different shapes appear `at the outputs of said `filtering means; adjustable 'time delay means Vfor relatively delaying thesignals applied 'to said filters fby such `amounts -as tto `ln'irig selected -parts yof `the 'lilter'ed outputs into phase with teach other; means for combining Athe outputs of vsaid iiltr-:ring means; `and means for 'recording the combined outputs as a'iunctioniof time.

15. .In apparatus `for seismic prospecting; a vgeop'hone Afor receiving seis'tnicfwaves arriving at a ;point in the earth and for converting said seismic wav-es tinto corresponding Velectrical waves; 4a :plurality 'of filtering means having dierent lfr'equencyeresp'onse characteristics; means for applying the electrical waves `=in parallel zto 'the inputs -of lsaid :filtering means, whereby filtered signals having `different shapes 4appear rat the /outputs '.of said ziiltering mea-ns; `adjustable time delay Ymeans forirelatively delaying the filtered signals by such amounts zas 'tofbring Selected `:parts yof the liiltered outputs into phase withieach other; :means Eor combining the 'outputs iof said Tfiltering meansgfmeans for reversing the .polarity uof said '-iilt'ereil outputs iprior yto combining; and means :for recording the combined outputs as 1a function iof'time.

References Cited in the file of this patent UNITED CSTAVDES PATENTS 2,`l421,8`l2 Richer a- Jan. 24, 19.3 2542-1421 `Reber d 4rsep'tfl, A1947 12,496,648 Wolf h- Feb. 7, i950 nassjsgsss McCarty mr Jiiiy 3, 1951 2,538,102 Lee May 12, 1953 A'2.,66lf4'6ll- `1).C3. 1l, li953 

1. IN APPARATUS FOR SEISMIC PROPECTING: A GEOPHONE FOR RECEIVING SEISMIC WAVES ARRIVING AT A POINT IN THE EARTH AND FOR CONVERTING SAID SEISMIC WAVES INTO CORRESPONDING ELECTRICAL WAVES; MEANS FOR TRANSLATING THE OUTPUT OF SAID GEOPHONE INTO A PLURALITY OF SIMILAR TRAINS OF ELECTRICAL WAVES THAT ARE DISPLACED IN TIME FROM EACH OTHER; A PLURALITY OF FILTERING MEANS HAVING DIFFERNET FREQUENCYRESPONSE CHARACTERISTICS; MEANS FOR APPLYING THE TIMEDISPLACED ELECTRICAL WAVES TO THE INPUTS OF SAID FILTERING MEANS; MEANS FOR COMBINING THE OUTPUTS OF SAID FILTERING MEANS; AND MEANS FOR RECORDING THE COMBINED OUTPUTS AS A FUNCTION OF TIME. 